Recovery of an anodically bonded glass device from a susstrate by use of a metal interlayer

ABSTRACT

A structure and method for removing and recovering an anodically bonded glass device from a substrate using a metal interlayer interposed between the glass and the substrate is provided. As used in semiconductor mask fabrication, the structure comprises a silicon wafer substrate coated with a membrane on which a metal interlayer is disposed. The metal interlayer and a glass device are anodically bonded together. Recovery of the glass device is accomplished by chemically and mechanically removing the wafer and its membrane from the metal interlayer. The membrane is preferably removed using reactive ion etching to which the metal interlayer is resistant. The metal interlayer is then removed from the glass device using a highly corrosive chemical solution. The recovered glass device may then be reused.

TECHNICAL FIELD

The present invention generally relates to a structure and method forrecovering an anodically bonded glass device, and more particularly, toa structure and method for removing and recovering a glass deviceanodically bonded to a metal interlayer interposed between the glass anda substrate.

BACKGROUND OF THE INVENTION

X-ray masks used in semiconductor fabrication are typically manufacturedby first forming a membrane on a silicon wafer, followed by anodicbonding of the coated wafer to a sodium-containing borosilicate glassring for additional support and rigidity. However, the anodic bondingprocess is irreversible, and any masks or mask blanks that fail duringprocessing are discarded without recovering the bonded glass supporttherefrom. Sodium-containing borosilicate glass is used for the supportring in the semiconductor mask industry because its thermal expansioncoefficient closely matches that of silicon. It is also advantageousbecause of its high chemical resistance. However, the cost ofsodium-containing borosilicate glass is an expensive component in thecost of the mask fabrication. Thus, discarding the glass increases themanufacturing cost of semiconductor masks.

Anodic bonding is a process for sealing certain easily oxidizable metalsto certain glass devices in which no adhesive is necessary. Contact madebetween the materials at high temperatures and with electrical biascreates an irreversible hermetic seal between the materials. In additionto semiconductor mask manufacturing, anodic bonding is often used byother industries where a hermetic seal is required between appropriatemetals and glasses. Examples of such applications include use inmultipurpose sensors such as semiconductor pressure sensors, pressuretransducers, acceleration sensors, and vibration sensors.

However, because neither reverse biasing nor cooling will break theanodic bond, no recovery of the glass is currently possible usingavailable methods if the substrate to which it is bonded should failduring subsequent processing. Previous efforts directed toward therecovery of the glass support ring from semiconductor mask manufacture,for example, have been unsuccessful.

Mechanical polishing of the substrate surface to remove the substratefrom the bonded glass device is generally unsuitable because suchmachining will also remove a portion of the glass. Similarly, strongacid or base solutions such as potassium hydroxide orhydrofluoric/nitric/acetic acid solutions used to remove the metalsubstrate will also damage the glass device. Generally,sodium-containing borosilicate glass, the glass commonly used in thesemiconductor industry, recovered in this way cannot be remachined orpolished for subsequent use due to thickness tolerances on the glassring support.

A need therefore exists for a practical method and structure for therecovery of glass anodically bonded to a substrate without damaging theglass upon removal of the substrate from it. In particular, in thesemiconductor mask manufacturing industry, a need exists for a structureand method which permits the reuse of the recovered sodium-containingborosilicate glass in manufacturing new masks.

Disclosure of the Invention

Briefly, in one aspect of the present invention, a structure is providedfor facilitating the recovery of a glass device, preferably comprising asodium-containing borosilicate glass, contained therein without damagingthe glass device. The structure comprises a substrate having a metalinterlayer interposed between it and the glass device. In a morespecific embodiment, the substrate has a film on a surface thereof, andthe metal interlayer is disposed on the film.

In another aspect, the present invention provides a method for theremoval and recovery of an anodically bonded glass device, preferablysodium-containing borosilicate glass, from a metal interlayer on asubstrate where the metal interlayer anodically bonds with the glassdevice. In a more specific embodiment, a film is interposed between thesubstrate surface and the metal interlayer. The method comprisesmechanically or chemically removing the substrate, and the film whereone is used, from the metal interlayer, followed by removing the metalinterlayer from the glass device with a chemical solution which iscorrosive to the metal interlayer but which does not damage theunderlying glass device.

The metal interlayer is a layer of an easily oxidizable metal used tofacilitate anodic bonding of the metal interlayer to the glass device.Preferably, the metal interlayer is resistant to selective reactive ionetching (RIE), thereby facilitating removal of a material that isetchable by RIE without removing the metal interlayer. An etch-rateratio (ERR) of 2:1 or greater between the material and the metalinterlayer is usually necessary. A typical ERR for silicon to chromiumis 50:1 in sulfur hexafluoride chemistry. As used herein, the term"etchable by RIE" refers to being removable by etching at a ratetypically greater than about 1000 Å/minute. The term "resistant to RIE"as used herein refers to being removable by etching at a rate typicallyabout 500 Å/minute or less.

The glass device is generally susceptible to RIE damage, and the metalinterlayer acts as an etch stop to prevent this damage to the glassdevice. In addition, the glass device is chemically inert to theotherwise corrosive chemical solutions used to react with and remove themetal interlayer from the glass. As used herein, the term "chemicallyinert" refers to being nonreactive with and left undamaged by corrosivechemical solutions. The term "corrosive" is used herein to refer tobeing capable of reacting with and removing by dissolution a materialfrom another material without leaving a residue.

To restate, the present invention satisfies the need for a structure andmethod for removing and recovering an anodically bonded glass devicefrom a substrate without damaging the glass device. It accomplishes thisby introducing a removable metal interlayer between the glass and thesubstrate. For example, the present invention provides a structure andmethod for recovering the sodium-containing borosilicate glass supportring used in semiconductor mask fabrication which permits reuse of therecovered glass if the original mask fails during processing subsequentto bonding. Because the expensive glass support ring is a significantcost component in the final assembly of the semiconductor mask, and ringusage is substantial, an economic advantage to the semiconductor maskindustry can be realized by practicing the present invention, whichenables the glass to be reused if the mask is discarded.

In addition, the structure of the present invention is advantageous overcurrent structures used in the fabrication of semiconductor masksbecause it permits use of a substrate that is not capable of anodicbonding to glass. Current mask structures comprise glass directlyanodically bonded to silicon, silicon oxide, silicon carbide, or siliconnitride. However, a carbon (diamond) membrane layer on a silicon waferdoes not anodically bond to glass, and a layer of silicon oxide isnecessarily deposited onto the carbon (diamond) membrane to facilitateanodic bonding of the mask to glass. Using the structure of the presentinvention, wherein a metal interlayer is interposed between thesubstrate and the glass prior to anodic bonding, deposition of a siliconoxide coating onto the carbon (diamond) membrane is not necessary. Thus,any substrate that adheres to the metal interlayer can indirectlyanodically bond to glass.

Particular applications in semiconductor mask fabrication in which thestructure and method of the present invention may be used include, butare not limited to, ion-beam projection masks, electron-beam projectionmasks, X-ray masks, and Scattering with Angular Limitation ProjectionElectron Lithography (SCALPEL) masks (See J. A. Liddle & S. D. Berger,SCALPEL Masks, 14th Annual Bacus Symposium (Sept. 1994)). In addition,the structure and method of the present invention may be employed inmicromachining applications such as sensor packaging andglass-encapsulated relays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an embodiment of the structure of thepresent invention;

FIG. 2 is a cross section of a preferred embodiment of the structure ofthe present invention;

FIG. 3 is a cross section of a conventional X-ray mask; and

FIG. 4 is a cross section of an X-ray mask illustrating a preferredembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As noted, the present invention provides a structure and method forremoving and recovering an anodically bonded glass device from asubstrate without damaging the glass device. In particular, anoxidizable metal interlayer is formed on the surface of the substrate,and the glass device is anodically bonded thereto. The substrate ismechanically or chemically removed from the metal interlayer withoutdamaging the underlying glass device. The metal interlayer is thenremoved from the glass device using a corrosive chemical solution thatreacts with the metal interlayer but does not attack the glass device,which is therefore available for subsequent use.

As shown in FIG. 1, one embodiment of the structure of the presentinvention comprises a substrate 12 which has a surface 13 coated with anoxidizable metal interlayer 14 which is then anodically bonded to aglass device 16. Any substrate that will adhere to the particular metalinterlayer may be used, and substrate need 12 not be capable of anodicbonding itself.

In a preferred embodiment, as shown in FIG. 2, a film 18 is interposedbetween substrate 12 and metal interlayer 14. Film 18 is necessarily onat least the bottom surface 13 of substrate 12, and is coated withoxidizable metal interlayer 14 which is anodically bonded to glassdevice 16. For example, a cross section of the preferred embodiment ofthe structure in semiconductor X-ray mask fabrication is shown in FIG. 4where substrate 12 is typically a silicon semiconductor wafer having afilm 18 comprising a membrane material on the entire wafer substratesurface including both the top surface 22 and bottom surface 13 of wafersubstrate 12. In X-ray mask manufacturing, membrane material film 18 isan X-ray permeable, chemically durable membrane layer such asboron-doped silicon, silicon carbide, silicon nitride, carbon (diamond),boron nitride, beryllium, and polysilicon. In other applications, suchas in ion-beam projection masks and electron-beam projection masks, themembrane material used to form film 18 may be permeable or opaque toelectrons, or opaque to ions. Use of the term "opaque" herein refers toblocking the passage of particles such as ions or electrons through themembrane material.

In semiconductor X-ray mask manufacturing, the industry currently usesthe structure shown in FIG. 3 as a cross section of a conventional X-raymask. The bottom surface of membrane material film 18 on the wafersubstrate 12 is anodically bonded to glass support 16. By contrast, FIG.4 which shows an X-ray mask cross section according to the preferredembodiment of the present invention includes metal interlayer 14interposed between membrane material 18 disposed on wafer substrate 12,and glass device 16.

Metal interlayer 14 comprises an easily oxidizable metal capable ofanodically bonding to glass device 16. In addition, the metal used toform metal interlayer 14 must adhere well to membrane material film 18or substrate 12 and must be easily removable from glass device 16 towhich it is anodically bonded such that the removal doesn't damage theglass. Generally, the metal used is reactive with corrosive chemicals towhich glass device 16 is chemically inert, while at the same time, themetal is resistant to chemical reaction during processing of thesubstrate. Examples of metals that may be used to form metal interlayer14 include, but are not limited to, chromium, aluminum, hafnium, nickel,and iron.

Glass device 16 used in the structure of the present invention may becomprised of any glass which anodically bonds to metal interlayer 14.The glass used should have a thermal expansion coefficient closelymatched to that of the substrate 12 material to which it is anodicallybonded in order to prevent stressing between the materials upon heatingand cooling which would break the anodic bond. In semiconductor maskmanufacturing, the glass is preferably any sodium-containingborosilicate glass such as Corning 7740 Pyrex™ or its equivalent becauseits thermal expansion coefficient closely matches that of silicon. Inaddition, sodium-containing borosilicate glass is resistant to chemicalattack. As shown in FIGS. 3 and 4, the configuration of glass device 16in semiconductor X-ray mask fabrication used to support wafer substrate12 and membrane film 18 is typically in the shape of a ring with anaperture 20 in the center thereof.

The method for removing and recovering an anodically bonded glass device16 from substrate 12 in accordance with the present invention includesforming the above-mentioned structure. In semiconductor maskmanufacturing, for example, the silicon wafer substrate 12 is coatedwith membrane material film 18 using conventional techniques including,but not limited to, chemical vapor deposition, diffusion, sputtering, orevaporation.

Metal interlayer 14 is deposited onto membrane material film 18 whereone is formed (FIGS. 2 and 4), or directly onto substrate 12 (FIG. 1)using conventional techniques such as evaporation or sputtering, forexample. Areas of membrane material film 18 or substrate 12 can beprotected against unwanted deposition of metal interlayer 14 by using aphysical mask to cover such areas, or the deposited interlayer 14 maylater be removed by etching the metal from the areas. Glass device 16 isthen anodically bonded to metal interlayer 14 using known methods atappropriate temperatures and voltages.

Removal and recovery of glass device 16 from the structure of thepresent invention generally comprises chemically or mechanicallyremoving substrate 12 and membrane material film 18 from metalinterlayer 14, followed by the removal of metal interlayer 14 using acorrosive chemical solution to which glass device 16 is chemicallyinert.

In the preferred embodiment, such as that used in semiconductor X-raymask manufacture and shown in FIG. 4, membrane material 18 on topsurface 22 of silicon wafer substrate 12 along with the bulk of siliconwafer 12 may be removed mechanically using commonly known methods suchas polishing or grinding. In addition, where glass support 16 is in theshape of a ring, the center of the wafer that is not supported by thering may also be removed by grinding or polishing. The remainingthickness of silicon wafer substrate 12 can then be removed chemicallyusing known liquid silicon etchants which do not react with underlyingglass support ring 16. For example, solutions ofethylenediamine/pyrocatechol/water or ethanolamine/gallic acid/water maybe used to remove the remaining silicon.

Preferably, as in semiconductor masks and shown in FIG. 4, for example,the bottom surface membrane material film 18 is removed from metalinterlayer 14 using selective reactive ion etching (RIE) techniques. Themembrane material film 18 is typically RIE etchable, which makes RIE aparticularly good method for its removal. RIE chemistry should be chosenthat will etch the particular membrane at a high rate. For example,where an X-ray permeable membrane adhered to the metal interlayer iscarbon (diamond), oxygen-based RIE chemistry may be employed to etch themembrane. Fluorine-based RIE chemistry is generally used to remove otherX-ray permeable membranes such as boron-doped silicon, silicon carbide,or silicon nitride.

In addition, when RIE is employed to remove membrane material film 18,underlying metal interlayer 14 must be resistant to RIE to avoiddamaging underlying glass device 16, which is susceptible to RIE damage.Thus, metal interlayer 14 acts as an etch stop for the chosen RIEchemistry to prevent etching the glass. The interlayer metals listedabove are suitable etch stop materials and may be used to form the metalinterlayer 14 in conjunction with the method of RIE to remove membranematerial film 18. However, the method of the present invention is notlimited to the use of a metal interlayer 14 that is resistant to RIE,and any chemical or mechanical means for removing the material adheringto the metal interlayer 14 and the metal interlayer 14 itself may beused so long as such method does not damage the underlying glass device16.

The remaining metal interlayer 14 bonded to glass device 16 is removedfrom the glass using a corrosive chemical solution to which theunderlying glass is chemically inert. For example, solutions of cericammonium nitrate or fuming sulfuric acid are chemicals known to reactwith oxidizable metals leaving sodium-containing borosilicate glassunaffected. However, the invention is not limited to the use of theaforementioned corrosive chemicals, and additional corrosive chemicalsthat may be used to remove metal interlayer 14 will be obvious to thoseskilled in the art.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that other changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

We claim:
 1. A method allowing the recovery of a glass device comprisingthe steps of:a) anodically bonding a glass device to an oxidizable metalinterlayer overlying a substrate; (b) removing said substrate from saidmetal interlayer; and (c) removing said metal interlayer from said glassdevice by contacting said metal interlayer with a chemical whichcorrodes said metal interlayer and toward which said glass device ischemically inert.
 2. The method according to claim 1, wherein said metalinterlayer is a layer of metal selected from the group consisting ofaluminum, chromium, hafnium, nickel, and iron.
 3. The method accordingto claim 1, wherein said chemical which corrodes said metal interlayeris selected from the group consisting of ceric ammonium nitrate andfuming sulfuric acid.
 4. The method according to claim 1, wherein saidglass device comprises a sodium-containing borosilicate glass device. 5.The method according to claim 1, wherein step (b) comprises the methodof selective reactive ion etching.
 6. The method according to claim 1,wherein said substrate comprises a body selected from the groupconsisting of silicon, silicon oxide, silicon nitride, and siliconcarbide.
 7. The method according to claim 1, wherein said substratecomprises:(a) silicon; and (b) a membrane material film interposedbetween said silicon and said metal interlayer.
 8. The method accordingto claim 7, wherein said membrane material film comprises a materialselected from the group consisting of membrane materials that arepermeable to X-rays, permeable to electrons, opaque to electrons, andopaque to ions.
 9. The method according to claim 7, wherein saidmembrane material film comprises a material selected from the groupconsisting of boron-doped silicon, silicon carbide, silicon nitride,carbon (diamond), boron nitride, beryllium, and polysilicon.